Basis of presentation and significant accounting policies - Divestitures (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset Disposition, Estimated Selling Price	$ 438,000,000
Restricted deposit on pending sale	44,000,000		44,000,000		0
Accounts receivable, net	579,000		579,000		416,000
Deferred income taxes	0		0		0
Other current assets	0		0		0
Total current assets held for sale	579,000		579,000		416,000
Total property and equipment held for sale, net of accumulated depreciation	45,698,000		45,698,000		44,812,000
Other assets, net	389,000		389,000		4,480,000
Total noncurrent assets held for sale	389,000		389,000		4,480,000
Accounts payable	726,000		726,000		1,112,000
Undistributed revenue and royalties	5,377,000		5,377,000		4,077,000
Other current liabilities	9,000		9,000		707,000
Total current liabilities held for sale	6,112,000		6,112,000		5,896,000
Asset retirement obligations	7,866,000		7,866,000		7,510,000
Other noncurrent liabilities	1,550,000		1,550,000		1,040,000
Total noncurrent liabilities held for sale	9,416,000		9,416,000		8,550,000
Revenues:
Natural gas transportation and treating	1,478,000	909,000	3,310,000	2,245,000
Total revenues from discontinued operations	1,478,000	909,000	3,310,000	2,245,000
Cost and expenses:
Natural gas transportation and treating	433,000	377,000	969,000	634,000
Drilling and production	236,000	64,000	480,000	285,000
Depreciation, depletion and amortization	0	634,000	627,000	1,248,000
Total costs and expenses from discontinued operations	669,000	1,075,000	2,076,000	2,167,000
Income (loss) from discontinued operations before income tax	809,000	(166,000)	1,234,000	78,000
Income tax (expense) benefit	(291,000)	60,000	(444,000)	(28,000)
Income (loss) from discontinued operations	518,000	(106,000)	790,000	50,000
Pipeline and gas gathering assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total property and equipment held for sale, net of accumulated depreciation	44,437,000		44,437,000		43,524,000
Other fixed assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total property and equipment held for sale, net of accumulated depreciation	$ 1,261,000		$ 1,261,000		$ 1,288,000